Study On Acetic Acid Synthesis From Methane And Carbon Dioxide Over Zirconium-based Catalysts

The conversion and utilization of greenhouse gas methane（CH₄）and carbon dioxide（CO₂）is of great significant to the survival and development.In addition,as the raw materials for chemical industry,CH₄and CO₂exhibit the merits of easy accessibility and low cost which could create a great deal of economic value for their conversion into high value products.It is industrial considered to convert them into syngas（CO+H₂）via dry-reforming process and then produce valuable chemicals through Fischer–Tropsch process or methanol synthesis using syngas as feedstock.Due to the inertness of both CH₄ and CO₂,the reforming process usually requires a high temperature of 800°C with the consumption of huge energy.Also,the process faces the undesired but severe carbon deposition,leading to the poor catalytic stability.Therefore,it deserved to pay more attention to the research of co-conversion of CH₄ and CO₂into acetic acid under a milder condition.Acetic acid is an important raw material for industry.Its synthesis from CH₄ and CO₂ which is cheap but bad for environment could improve the environment and also cut the cost for acetic acid production.Moreover,this process exhibits a100%atomic efficiency,which is in line with the concept of green chemistry.However,owing to the chemical stability of CH₄and CO₂,for the co-conversion of them,it is necessary and effective to design and construct a catalyst which could activate the inert CH₄and CO₂efficiently.In this paper,sulphuric acid and Palladium modifying ZrO₂ catalysts are constructed for the co-conversion of CH₄ and CO₂ into acetic acid.Relevant experiments,characterizations and DFT calculations are performed to elucidate the influence of modification on acetic acid formation.（1）Sulfated zirconia with different sulfur content as well as pure ZrO₂ catalysts are constructed for acetic acid synthesis.Pure ZrO₂ shows limited acetic acid production.However,as for sulfated zirconia,the increment of acetic acid production is achieved with the increase of S content.Relevant characterizations and DFT calculations suggest that Lewis acid sites show stronger ability for CH₄ activation than Bronsted acid sites.With the increase of Lewis acidity,the barrier of CH₄ dissociation decreases.In addition,CO₂ molecules show stronger interaction with catalysts when the intensity of Lewis acid increases.The pathway for acetic acid formation is calculated via DFT calculations.First,CH₄and CO₂ are activated on Lewis acid sites to form CH₃*,H*and CO₃*.Then,CO₃H*is formed via CO₃*hydrogenation,which exhibit a lower energy barrier for the C-C coupling with CH₃*to form acetic acid,owing to a weakening C-O bond from the hydrogenation.（2）We construct the catalysts through adding different metals into ZrO₂ for acetic acid synthesis and also pretreat the catalysts with H₂.After screening,Pd modifying ZrO₂ catalysts show the best performance for acetic acid formation.With the increase of Pd loadings,TOF for acetic acid formation increases.In addition,the pretreatment of H₂ also facilitates acetic acid formation.Further explements indicate that acetic acid originates from the coupling of CH₄and CO₂,owing to pure CH₄ or CO₂ could not from acetic acid on Pd-ZrO₂ catalysts.Relevant characterizations indicate that the addition of Pd could promote the reduction of ZrO₂ to generate oxygen vacancies and-OH,enhancing CO₂ adoption amount obviously.Meanwhile,the new formed Pd-ZrO₂ interface shows strong interaction with CO₂.Moreover,after H₂pretreatment,modifying ZrO₂ with higher Pd loadings could form Pd nanoparticles with larger size.Relevant experiments prove that TOF of CH₄ increase with Pd particle size increasing.The pathway for acetic acid formation over Pd-ZrO₂ catalysts is also computed via DFT calculations.CH₄ dissociation occurs on Pd interface to generate CH₃*and H*.At the same time,CO₂ adsorbs on Pd-ZrO₂ interface to form CO₂*.Then,CO₂H*is formed via hydrogenation,which exhibit a lower coupling barrier with CH₃*to form acetic acid,owing to the same reason in the case of sulfated zirconia that the hydrogenation of CO₂*could weaken the C-O bond and then favor the coupling process.It is worth noting that the calculated barrier of CH₄ dissociation and C-C coupling over Pd-ZrO₂ is much lower than the case in sulfated zirconia,indicating that Pd modifying ZrO₂ exhibit better performance for the synthesis of acetic acid from CH₄ and CO₂.